1. Field of the Invention
The present invention relates to an abnormality diagnosis device that performs abnormality diagnosis of an intake air quantity sensor (an airflow meter) that senses intake air quantity of an internal combustion engine.
2. Description of Related Art
As a conventional abnormality diagnosis device of an intake air quantity sensor (an airflow meter), there is an abnormality diagnosis device that determines existence/nonexistence of an abnormality in an intake air quantity sensor based on whether a difference between a sensing value of the intake air quantity sensor (i.e., actual intake air quantity) and target intake air quantity is equal to or greater than a predetermined determination threshold value when a throttle opening degree is equal to or smaller than a predetermined throttle opening degree (i.e., in a small air quantity range where the actual intake air quantity is not fluctuated by fluctuation of engine rotation speed) during an operation of an internal combustion engine, for example, as described in Patent document 1 (Japanese Patent No. 3052813).
The abnormality diagnosis device of the intake air quantity sensor described in Patent document 1 performs the abnormality diagnosis of the intake air quantity sensor based on whether the difference (i.e., the error) between the sensing value of the intake air quantity sensor (i.e., the actual intake air quantity) and the target intake air quantity exceeds the determination threshold value (i.e., a permissible error) while there occurs a steady operation state where the actual intake air quantity is small, Therefore, when a response characteristic of the intake air quantity sensor is deteriorated by degradation of a sensor element, a circuit failure or the like, the deterioration of the response characteristic cannot be detected as the abnormality. It is because time until the sensing value of the intake air quantity sensor converges to the target intake air quantity merely extends and the difference between the sensing value of the intake air quantity sensor and the target intake air quantity falls within the determination threshold value when the time elapses in the steady operation state, in which the abnormality diagnosis of Patent document 1 is performed, even if the response characteristic of the intake air quantity sensor deteriorates.
If the response characteristic of the intake air quantity sensor deteriorates, time since the engine is started until accurate sensing of the intake air quantity becomes possible lengthens largely or sensing accuracy of the intake air quantity during a transient time in which the intake air quantity fluctuates worsens. In such the case, control accuracy of the air-fuel ratio control after the engine start or the air-fuel ratio control during the transient time deteriorates, and problems such as emission deterioration will arise.